The present invention relates to an improved composition of matter having absorbent characteristics. In particular, this invention is directed to an improved water-swellable extended absorbent composition of matter, processes for the preparation thereof, and absorbent articles prepared therefrom.
Recently there has been increased activity in the area of water-insoluble particulate hydrocolloid absorbent compositions of matter and products using the same, such as absorbent dressings, diapers, catamenial tampons, and the like for absorbing aqueous fluids such as water, urine, blood and other aqueous body exudates. Such substantially water-insoluble compounds maintain their particulate character as they imbibe and absorb several times their weight of surrounding liquid. In doing so, each individual absorbent particle swells or enlarges several hundred percent its individual perimeter without destruction of its initial particulate integrity. Each particle maintains the approximate shape and geometry it had before contact with liquid, but the dimensions are greatly enlarged to provide for the binding of the liquid absorbed therein. The gelatinous mass of swollen particulate water-insoluble particles defines an aciniform structure since each individual absorbent particle is a greatly enlarged particle, having become liquid-swollen or grape-like or acinus in form due to the liquid it has absorbed. The individual swollen particles are tacky and hence form a clustered mass of liquid-swollen particles. The particles remain in an acinus form state despite the presence of liquid in excess of their ability to absorb. The liquid-swollen particles bind their absorbed water tightly but upon drying they become dehydrated, returning more or less to their original size, and can operate substantially as before to absorb and bind liquids.
The water-insoluble absorbent compositions described above are generally formed in either of two ways. As described in U.S. Pat. Nos. 3,628,534; 3,669,103 and 3,670,731, one or more monomers, which if homopolymerized would form a water-soluble polymer, are copolymerized with a polyfunctional monomer which covalently crosslinks the molecule and introduces a limited water insolubility. In general, the degree of crosslinking is controlled so that the polymer is not soluble in aqueous media, yet remains flexible and swells as the aqueous media is absorbed within its structure.
Alternatively, as described in U.S. Pat. No. 4,090,103, such water-insoluble compositions may be formed through the polymerization and ionic complexing of one or more monomers, which if homopolymerized would form a water-soluble polymer, with polyvalent metal cations having a valence of at least three. The advantage of the ionically complexed compositions over the covalently crosslinked compositions is that the former are easy to shape and apply to substrates for particular applications since they may be uncomplexed and recomplexed by adjusting the pH.
Another class of materials known for their good water-absorbent properties is the polysaccharide graft polymers as described in such patents as U.S. Pat. Nos. 2,922,768 and 4,134,863.
Clay is known for its property of absorbing aqueous liquids; however, its colloidal, dispersive characteristics in water prevent its use in such absorbent materials as tampons or disposable diapers. It is known from U.S. Pat. No. 3,935,363 that clay minerals, when flocculated into granular aggregates using small amounts of an inorganic salt solution and/or a water-soluble polymeric flocculating agent such as polyacrylic acid, and then dried, have enhanced water-absorbing properties. However, the absorbency effect of clay and/or soluble polyelectrolytes when blended with a water-insoluble absorbent material has not been disclosed in the prior art.
U.K. Pat. No. 1,554,002 discloses sanitary articles for absorbing body fluids containing as the absorbent composition a mixture of an acrylic acid polymer salt and guar gum, alginate, xanthan gum or mixtures thereof, but the teaching is limited to these three classes of materials.
Accordingly, it is an object of the present invention to provide an improved dry, solid, water-swellable extended composition of matter which is economical to prepare and which mainfests good or dramatically improved absorbent properties.
It is another object to provide processes for preparing such a composition and an absorbent article prepared from the composition.